The present invention relates to a method and a device for calibrating an imaging optical unit for metrological applications. In particular, the invention relates to a method and a device making use of an electronic display for calibrating an imaging optics of a measuring machine.
DE 10 2006 034 350 A1 discloses a calibration piece for producing bright-dark transitions with a known distance from one another. The calibration piece is an elongate cuboid bar having a plurality of through-holes, the opening diameters of which at the upper side and the lower side of the calibration piece differ. Some of the through-holes taper from the upper side of the calibration piece toward the lower side. Other through-holes taper from the lower side toward the upper side. Incorrect measurements due to isolated edges and distortion due to a lack of parallelism are supposed to be avoided on the basis of this calibration piece. The known calibration piece in particular serves for calibrating a line scan camera, with the aid of which the edge position of material webs passing below the line scan camera is determined.
DE 10 2004 020 881 A1 discloses a method and a device for geometrically calibrating a measurement image camera, wherein a defined test structure is generated using a coherent, monochromatic light source and a double slit. The double slit is illuminated, for example, by a light source in the form of a laser. The two slits of the double slit each produce a diffraction pattern, which is recorded with the measurement image camera to be calibrated.
DE 195 36 297 A1 describes a method for geometrically calibrating optical 3D sensors, wherein a special calibration body is used, which is configured to have a black and white striped pattern. The pattern is arranged orthogonally with respect to a stripe pattern which the optical 3D sensor requires for the measurement. The calibration body can have further signal markings or reference markings which must be within the camera's field of view.
The known calibration devices and methods each use special calibration bodies having known properties which are specifically matched to the measurement device to be calibrated. The more accurate and extensive the calibration is intended to be, the greater is the complexity for the provision of the required calibration bodies. On the other hand, there is an increasing demand for being able to use imaging optical units for metrological applications in a highly flexible manner and accordingly for calibrating with respect to a plurality of different aberrations. Multifarious measurement tasks arise in this context, which are intended to be optimized in the one or the other direction, depending on the size, type and nature of the workpieces.
DE 10 2010 000 745 A1 describes a calibration pattern for calibrating an image measurement instrument and in particular for calibrating a camera-based measurement device for measuring the length of a measurement object. The calibration pattern consists of concentric or helical square rings which provide bright-dark transitions that alternate in the x and y directions. The respective distances of successive bright-dark transitions and/or dark-bright transitions are known and serve for calibrating the optical measurement device. The respectively appropriate transitions in the calibration pattern can be used for the calibration in dependence on the appearance of the bright-dark transitions on a measurement object to be measured. DE 10 2010 000 745 A1 fails to describe how the proposed calibration patterns are to be implemented in practice. However, the fact that the actual distances between the bright-dark transitions and/or dark-bright transitions are intended to be used for the calibration implies that said distances must be produced with great accuracy and reproducibility, which in turn implies a calibration body that is produced specifically for this calibration.